2. Varieties.—Hornblende occurs in trap rocks, either alone or mixed with other minerals, and having different structures; and this arrangement affords a good basis for their subdivision. In the oldest trap, no iron-clay occurs; it first makes its appearance in the transition period, and increases in the newer periods.
Primitive Trap, in particular, is almost always dis tinguisl.led by a great predominance of hornblende, so that some of the kinds are wholly or almost entirely composed of hornblende. This character affords the first sub-division of primitive trap.
There are three principal species of primitive trap, and these again have their subordinate kinds.
The following table exhibits the rocks of this series.
1. Common hornblende rock.
a. Granular hornblende rock.
b. Hornblendc-Slate.
2. Hornblende mixed with felspar.
a. Greenstone. Diabase.—Brongniart.
a. Common Greenstone.
p. Porphyritic Greenstone.
Grcenstone Porphyry.
.9phanite.—Hauy.
'. Green Porphyry.
b. Greenstone Slate. Diabase schisteuse.
3. Hornblende mixed with mica.
The three principal species are, 1. Common Horn blende-rock ; 2. Hornblende mixed with felspar ; and, 3. Hornblende mixed with mica.
I Common Hornblende-rock is almost entirely corn posed of Hornblende. It contains two subordinate kinds ; the first is denominated Granular Hornblende-rock; the second, which differs from the first only in having a slaty structure, is denominated Hornblende-slate. It passes sometimes into gneiss, and sometimes into chlo rite-slate, and often into hornblende-rock. These two rocks occur in beds, in gneiss, mica-slate, and clay-slate, but the beds are thicker and more numerous in the clay slate than in mica-slate or gneiss.
It occurs in the islands of Arran, Coll, and Time; also in the district extending from Loch Lomond to Dunkeld, and many ether places of the Highlands of Scotland. It abounds also in Bohemia, Saxony, the Tyrol, Siberia, and many other countries.
2. Hornblende mixed with Felspar.—This species con tains two subordinate kinds ; the first is Greenstone, the second Greenstone slate.
(1.) The Greenstone comprehends the following varie ties: Common Greenstone, Porphyritic Greenstone, Green stone-Porphyry. and Green Porphyry.
a. Common Greenstone is a granular aggregate of hornblende and felspar. b. Porphyritic Greenetone is
the preceding kind, including large crystals of felspar, and consequently having a porphyritic structure. c. Greenstone Porphyry. In this variety the granular basis, which is with difficulty distinguishable, includes crystals of felspar. It is the Black Porphyry of the ancients. d. Green Porphyry. In this variety the granular nature of the basis is no longer visible to the naked cyc ; it appears uniform and simple ; has a blackish green or pistachio green colour, and includes crystals of compact felspar. It is the Porfire verte, or antique green por phyry of antiquaries. The Variolite probably belongs to the green porphyry.
Greenstone appears sometimes stratified. Its different varieties first appear in gneiss, then in mica-slate, and lastly in clay-slate. In mica-slate, but more particularly in gneiss, the beds are few and inconsiderable ; whereas in clay-slate they are numerous and of great magnitude. It probably, in some instances, occurs in an unconforma ble and overlying position, and hence may be sometimes considerably newer than clay-slate.
It occurs abundantly in this country. Thus the clay slate and mica-slate that form so great a portion of the country extending from Loch Lomond, by Callender, Comrie and Dunkeld, contain numerous beds of green stone; and there, as is the case in all other countries, the clay-slate contains more numerous and larger beds than the mica-slate. It is also very abundant on the continent of Europe, as Norway, Saxony, Bohemia, Silesia, Thuringia, Hungary, the Alps of Switzerland and Savoy.
Greenstone-alate is composed of hornblende and compact felspar, and has a distinct slaty structure. The felspar in general is rather more abundant than the hornblende. It sometimes contains scales of mica.
It occurs only in clay slate, and according to Werner is the newest of the primitive traps. It occurs in great beds, and even mountain-masses; so that in some coun tries, as Sweden, it is said to form ranges of hills. It is very metalliferous. The celebrated mining district of Gersdorf, in Saxony, is situated in this rock. The mining district of Rudolstadt in Silesia, and of Adelfors in Sweden, are also in greenstonc-slate.